Recently, in view of the environmental problems, requirements for lower fuel consumption for vehicle have been severer year by year, and size and weight reduction for vehicle parts is strongly required than before. With respect to this requirements for size and weight reduction, for example, researches are performed so much on high-strengthening of material and surface strengthening by surface treatment in a field of compression coil spring parts such as a clutch damper spring used in clutch or a valve spring used in engine, and as a result, fatigue resistance and settling resistance which are important as a property of coil spring have been improved.
Generally, a method for production of coil spring is roughly classified in a hot forming method and a cold forming method. The hot forming method is employed in forming coil spring in which cold forming is difficult due to its low workability, such as a coil having large wire diameter d, and having small spring index D/d which is a ratio of coil average diameter D and the wire diameter d. As such a coil spring material, carbon steel and spring steel are mentioned. In the hot forming method, as shown in FIG. 1F, wire material is heated to high temperature so as to be processed easily, wound around core metal so as to perform coiling in a coil spring shape, quenched, tempered, and further processed by shotpeening or setting, so as to obtain fatigue resistance and settling resistance which are important as a property of coil spring. It should be noted that in the hot forming process, coiling without using core metal is not practically realized since it is technically very difficult. Therefore, in a conventional technique, core metal is necessary in hot forming method, and a coil spring which can be produced in the method has lower degree of freedom in shape compared to a coil produced by the cold forming method in which coiling can be performed without using core metal.
On the other hand, with respect to compression coil spring of valve spring or clutch damper spring class, the cold forming can be employed because they have comparatively small wire diameter. In addition, high dimension accuracy is easily obtained since there is no transformation or thermal expansion and shrinkage by heating. In addition, since mass production property (takt time, cost) by processing rate and facility cost is superior, compression coil spring in this class has been conventionally produced by the cold forming method. Furthermore, the cold forming method is employed mainly because forming technique without core metal is established in this cold forming method, and shape freedom degree of coil spring is high. A production technique of a compression coil spring of valve spring or clutch damper spring class by a hot forming method does not exist ever before. It should be noted that a hard drawn wire such as carbon steel wire, hard steel wire, piano wire and spring steel wire has been conventionally used as a coil spring wire material in the cold forming method. However, recently, from the viewpoint of weight reduction, high strengthening of material is required, and an expensive oil tempered wire is widely spreading.
In the cold forming method, as shown in FIGS. 1D and 1E, wire material is coiled in a coil spring shape in cold condition, annealed, and further processed by shotpeening and setting if necessary. Here, annealing has a purpose for removing residual stress which is generated by processing and which is obstructive factor for improving fatigue resistance of coil spring, and it may contribute for improving fatigue resistance of coil spring together with imparting of compressive residual stress onto surface by shotpeening. It should be noted that with respect to a coil spring used in heavy load stress such as valve spring or clutch damper spring, surface hardening treatment by nitriding treatment is performed if necessary before shotpeening.
Researches are performed so much on further improvement in fatigue resistance. For example, Japanese Patent No. 3595901 discloses an oil tempered wire for a cold forming, and a technique in which fatigue resistance is improved by using processing-induced transformation of residual austenite. Japanese Unexamined Patent Application Publication No. 2009-226523 discloses a technique in which multi-step shotpeening at different projection rates is performed onto surface of wire material on which nitriding treatment is performed so that large compressive residual stress is imparted and fatigue resistance is improved.
In the U.S. Pat. No. 3,595,901, residual stress may be generated in the coil spring after coiling. This residual stress, in particular tensile residual stress along wire axis direction generated on the surface of coil inner diameter side, is an obstructive factor for improving fatigue resistance as a coil spring. Ordinarily, annealing is performed so as to remove this residual stress by processing; however, it is easily estimated and is widely known for those in the art that it is difficult to completely remove this residual stress while maintaining desired wire material strength, even if the wire material having high tempering softening resistance disclosed in the U.S. Pat. No. 3,595,901 is used. Therefore, even if shotpeening is performed thereafter, it is difficult to impart sufficient compressive residual stress onto wire material surface due to influence of tensile residual stress by processing remained in coil inner diameter side, and sufficient fatigue resistance as a coil spring cannot be obtained. Furthermore, an element such as V or Mo which contributes for improving tempering softening resistance is expensive. Therefore, the coil spring as a product may also be expensive.
Furthermore, in the publication No. 226523, compressive residual stress in the vicinity of the wire material surface (hereinafter referred to as “surface”) of coil spring is about 1400 MPa, the compressive residual stress is sufficient for reducing cracking at the surface as a coil spring which is used under heavy load stress of valve spring or clutch damper spring class. However, as a result of improving compressive residual stress at the surface, compressive residual stress inside of the wire material is decreased, and the effect of the compressive residual stress against generating of cracking which starts from inclusions or the like inside of wire material, is poor. That is, since there is a limitation in energy imparted by shotpeening in the method of the publication No. 226523, it is difficult to greatly increase total sum of compressive residual stress although distribution of compressive residual stress can be changed so some extent. It is not considered to solve influence by the abovementioned residual stress by processing, therefore, effect of improving fatigue resistance for a wire material having same strength is poor.
It should be noted that kinds of means for improving surface compressive residual stress are practically realized, and as a result, in a coil spring having wire diameter of about 1.5 to 10 mm for example, the maximal value of synthesis stress which is a sum of action stress by outer load and residual stress exists in a range of 0.1 to 0.4 mm of depth from the wire material surface (hereinafter referred to as “depth”), and the part having the largest synthesis stress corresponds to an origin of breakage. Therefore, it is important for the fatigue resistance to maintain large compressive residual stress in a depth range of 0.1 to 0.4 mm.